The present invention relates to pre-expanded particles of a polypropylene resin suitable for the production of thermal insulation materials, cushioning packaging materials, returnable delivery boxes, automobile bumper cores and so on, and to a process for preparing polypropylene resin in-mold foamed articles from the pre-expanded particles.
Polypropylene in-mold foaming products are superior in chemical resistance, heat resistance and distortion restoration rate after compression as compared to polystyrene in-mold foaming products. They are also superior in heat resistance and compression strength as compared to polyethylene in-mold foaming products. For these reasons they are widely used as cushioning packaging materials, returnable delivery boxes, automobile bumper cores, and others.
In recent years, efforts to improve productivity are being made for the purpose of reducing the cost of industrial products in a wide range of fields, e.g., in the production of molded articles by shortening the molding time. It is strongly demanded to shorten the molding time also in the production of polypropylene in-mold foamed articles to reduce costs.
Various techniques for shortening the molding time are known in the production of in-mold foamed articles from polypropylene resin pre-expanded particles.
For example, Japanese Patent Publication Kokai No. 61-103944 discloses a process of the production of in-mold foamed articles using expanded particles of a non-crosslinked polypropylene having a density of 8 to 100 g/liter and a particle diameter of 1 to 5 mm and containing 0.02 to 1 part by weight of organic sodium phosphate per 100 parts by weight of the polypropylene. However, this process requires the incorporation of the phosphorus compound and, therefore, it may exert an unfavorable influence on environment and is also disadvantageous in cost. Further, when performing in-mold molding of the polypropylene expanded particles obtained, this process requires compressing the particles to 50%.
Japanese Patent Publication Kokai No. 8-20662 discloses a process for preparing molded articles wherein polypropylene resin 1 expanded particles exhibiting a high temperature side fusion peak, the heat of fusion of which is more than 3.5 cal/g and is not more than 6.0 cal/g, are compressed and filled in a mold in a compression rate of 10 to 60% and heated with steam to produce a molded article. In this process, however, the molding method is limited to only a specific method and there is a limit in shortening the molding time in general production of in-mold foamed articles.
It is an object of the present invention is to provide pre-expanded particles of a polypropylene resin which can be molded in a mold in a shortened molding time to give cellular molded articles.
A further object of the present invention is to provide pre-expanded particles of a polypropylene resin which can be stably molded according to various known in-mold foaming methods in a shortened molding time under a wide rage of molding conditions to give cellular molded articles having excellent properties such as weldability of particles and surface appearance.
Another object of the present invention is to provide a process for stably preparing polypropylene resin in-mold foamed articles having excellent properties in a shortened molding time.
The present inventors have found, as a result of making intensive study in view of such circumstances, that when performing in-mold foaming by using polypropylene resin pre-expanded particles, the expandability of which measured when heated by steam of a temperature equal to the melting point peak temperature of the base resin thereof measured by differential scanning calorimetry (hereinafter also referred to as xe2x80x9cDSC methodxe2x80x9d) is controlled within a specific range, the cooling time in the in-mold foaming can be shortened, and also that when a difference between the expandability when heated by steam of the melting point peak temperature and the expandability when heated by steam of xe2x80x9cthe melting point peak temperaturexe2x80x9410xc2x0 C.xe2x80x9d is controlled to not more than a specific range, the cooling time can be shortened stably even if the molding conditions such as heating pressure of in-mold foaming are changed, and further that pre-expanded particles can be suitably used in various in-mold foaming methods.
Thus, in accordance with the present invention, there is provided a pre-expanded particle of a polypropylene resin, the expandability of which shown by the equation:   Expandability  =                                          Expansion  ratio  of  polypropylene  resin  pre-                                                            expanded  particles  after  steam  heating                                                                    Expansion  ratio  of  polypropylene  resin  pre-                                                            expanded  particles  before  steam  heating                              
wherein the expansion ratio of polypropylene pre-expanded particles after steam heating denotes the value measured after the restoration from shrinking by impregnating the pre-expanded particles with air, is from 1.0 to 1.8 when heating the pre-expanded particles by steam of a temperature equal to a melting point peak temperature of its base resin measured by differential scanning calorimetry in such a manner as heating 1 to 10 mg of the pre-expanded particles to 210xc2x0 C. at a temperature elevation speed of 10xc2x0 C./minute in a differential scanning calorimeter, then cooling to 40xc2x0 C. at a temperature dropping speed of 10xc2x0 C./minute and heating again to 210xc2x0 C. at a temperature elevation speed of 10xc2x0 C./minute.
It is preferable that the polypropylene resin pre-expanded particles are those showing a difference of at most 0.2 between the expandability obtained when heating the pre-expanded particles by steam of the melting point peak temperature of the base resin and the expandability obtained when heating the pre-expanded particles by steam of xe2x80x9cthe melting point peak temperaturexe2x80x9410xc2x0 C.xe2x80x9d.
The present invention also provides a process for preparing polypropylene resin in-mold foamed articles characterized by imparting an inner pressure to pre-expanded particles of a polypropylene resin, the expandability defined above of which is from 1.0 to 1.5 when heating the pre-expanded particles by steam of the melting point peak temperature of the base resin, filling the particles in a mold capable of being closed but incapable of being hermetically sealed, and heating the particles by steam to fuse together.
Further, the present invention provides a process for preparing polypropylene resin in-mold foamed articles characterized by filling pre-expanded particles of a polypropylene resin, the expandability defined above of which is from 1.0 to 1.5 when heating the pre-expanded particles by steam of the melting point peak temperature of the base resin, in a mold capable of being closed but incapable of being hermetically sealed so that the compression rate is from 10 to 60%, and heating the particles by steam to fuse together.
Further, the present invention provides a process for preparing polypropylene resin in-mold foamed articles characterized by filling pre-expanded particles of a polypropylene resin, the expandability defined above of which is from 1.5 to 1.8 when heating the pre-expanded particles by steam of the melting point peak temperature of the base resin, in a mold capable of being closed but incapable of being hermetically sealed without applying an inner pressure-imparting procedure to the pre-expanded particles, and heating the particles by steam to fuse together.